Louisiana has many barrier islands along its coast.  These barrier islands provide habitats for many plants and animals, provide a barrier for hurricane storm surges, and contribute to Louisiana’s beauty, natural resources, and tourism. Barrier islands also play an important role in protecting and providing for Louisiana (as well as coastlines around the world). They act as natural speed bumps- slowing down storms, reducing storm surge, and protecting the coast from the intense wave action of the Gulf of Mexico. Many species live on or use barrier islands. For instance, on any given day you could see big groups of birds hanging out on a barrier island. The islands provide resting spots for migrating birds. The islands also are great places for some species to live and so the islands also have permanent plant and animal residents.

Formation of a Louisiana Barrier Island

Remember in Level 1 when we learned about the Delta Lobe Cycle? Barrier Islands begin as a delta lobe and lobes are formed when a river deposits sediments at the end of the river channel. When a river changes course, the lower rate of sediment being delivered cannot keep up with the rate of subsidence and sea level rise. Eventually, the part of the delta lobe behind the headland disappears and the headland becomes an island.

Physical Environment of Barrier Islands

Barrier islands can be separated into physical parts which are quite different from each other. Some of these areas have already been covered in other lessons and will have the same basic physical descriptions even though they are located on barrier islands.

• Foreshore (surf zone) – is the island’s side facing the ocean. Exposure to the ocean’s constant wind and wave action means only the heaviest particles are left behind which is sand. Because sand is easily moved this zone is constantly shifting and being turned over. Offers no solid surface to hold on to so organisms must be able to swim well or crawl rapidly to avoid being pounded by waves
• Backshore (beach) – is the sandy beach not typically covered by water, however, it can get some flooding from high waves or storm surges from hurricanes. This area has some pretty tough conditions. Wave action, shifting course sand, and periods of wet and dry conditions make it challenging for living things. Sand is blown by the wind from this area to the area behind it, the dune.
• Dunes – In Louisiana, dunes are smaller than dunes in some other places. This is because of large storms such as hurricanes along the Gulf Coast, which destroy the dunes. If dunes do remain intact, salt-tolerant vegetation will begin to populate the area, trapping even more sand and increasing the dune’s height and width.
• Washover fans – are made when large waves and storm surges break through the dunes and carry sand inland from the ocean-facing side to the backside of the island. Just like dunes, overwash fans are bare for a time, but salt marsh grasses and small shrubs can grow there if they remain undamaged by storms or high water events.
• Salt marsh – These marshes are a lot like the salt marshes we talked about earlier at this level. Remember the sediments in salt marshes have lots of dead plant material making a great place for plants like cord grass to grow since it has nutrients for the plants from past growing seasons and root material. Because they are closer to the bay and a bit lower in elevation these marshes do have water exchange daily during high tide.
• Mudflats – This area is on what is known as the low-energy side of the island. Covered only by a few inches of water at low tide and could be exposed during real low water events. The elevation of mudflats prevents the growth of salt marsh plants. This area tends to have very fine sediments like clay and silt but this will depend on the location of the mudflat. The amounts of the different sediments will determine what animals will be found there. Things living in the soils of mudflats can be exposed to low oxygen and even no oxygen conditions if the clay and silt form layers that oxygen from the atmosphere can not get through.

The Biology of Barrier Islands by Area

• Foreshore – Burrowing protects animals from wave energy and predators such as small fish. Two common burrowers are Acorn Worms and Ghost Shrimp. Both animals live most of their lives inside of the burrows and are filter feeders. Great place for interstitial meiofauna. Interstitial means between grains of sand. Meiofauna are animals that measure between 30 micrometers and 0.5 millimeters, meaning they are microscopic organisms. Common meiofauna include nematodes, or roundworms, and copepods which are crustaceans. Meiofauna feed on microalgae and bacteria and play a vital role in maintaining the base of the food chain in this environment.
• Backshore – If you remember this is a very harsh environment, but that does not stop living things from living there. There is typically little to no vegetation.  Some animals take refuge from sun exposure and predators under the lines of debris that wash up on the beach. However, many live beneath the sand’s surface which includes crustaceans (crabs), annelids (worms), and mollusks (clams). The backshore is also utilized by many species of birds where they hunt for food and sometimes nest. Sea turtles, like the Kemp Ridley, depend on the backshore habitat for nesting and mating. Because barrier islands can be remote there is some protection from predators like coyotes, raccoons, and others.  Once again, interstitial meiofauna is plentiful and directly supports life in this area.
• Dunes – could be bare with very little life, but just as likely could be crowded with grasses, small shrubs, and wildlife.  If dunes have not been damaged for a while plants such as Bitter Panicum, Marshhay Cordgrass, Gulf Bluestem, or Beach Morning Glory are common dune species on Louisiana barrier islands. Older dunes may include small shrubs as well including the Toothache Tree, Saltbush, Wax Myrtle, and Marsh Elder. Dunes function as an important nesting habitat for colonial waterbirds.  Colonial waterbirds are birds that form colonies.  These include Brown Pelicans, Roseate Spoonbills, and many others.
• Salt marsh – are low-lying and populated by salt-tolerant plants, microscopic organisms, and birds. When flooded, salt marsh hosts may larval fish and crustaceans.
• Mudflat – the types of living things can change depending on water exchange because of tides. When flooded this area hosts juvenile fish, shellfish, and snails. At low tide fiddler crabs, ghost crabs and mud shrimp can be found. Mudflats also serve as a nursery for fish and shellfish that move in and out as they reach adulthood. Wading birds can be found taking advantage of animals that they rely on for food. It is ideal for many birds because they don’t have to spend a lot of energy finding food in these areas. For instance, the American Oystercatcher can be found using its long thin bill for probing and foraging for food.